1. Field of the Invention
The present invention relates to primer cups and more particularly to primer cups useful with liquid or gelled blasting agents.
2. The Prior Art
It is well-known that ammonium nitrate-fuel oil combinations (ANFO) are comparatively inexpensive and effective blasting agents. However, ANFO is characteristically low in sensitivity and can be initiated only with an initiating explosion having a high detonation pressure. In recent years, water gels, slurries, and emulsions have been developed as blasting agents and are now widely used. However, these blasting agents, like ANFO, are low in sensitivity and historically have developed low detonation pressures. Accordingly, water gels, slurries, and emulsions require a primer and/or booster to initiate and propagate detonation. Thus, to initiate a blasting agent such as ANFO, water gels, and the like, it is common to first explode a primer or booster, having a high detonation pressure, in close proximity to the blasting agent.
In the past, solid explosives such as TNT, PETN, and Pentolite have been the preferred material for primers because of the high detonation pressure which they develop. Solid materials are well recognized to be both expensive and precarious. Because solid primer materials are precarious, there is undesirable hazard in handling, storing, and using the same.
Historically, solid primer materials have been formed into required shapes by casting them in cups. These cups are then used for the storage and handling of the more hazardous solid primer materials, and are referred to as primer cups.
Most recently, aqueous compositions developing sufficient detonation pressure for use as primers and boosters have been discovered. Conventional solid material-receiving primer cups have, however, been found inadequate for use with liquid and/or gelled aqueous explosives. In the past, primer cups have been designed for use with non-liquid primer material at ambient temperatures. Thus, conventional primer cups have not provided structure for complete sealing of the cup itself to secure the aqueous explosive therein.
Solid primers have been formed with an axial passage which permitted the through-insertion of detonators such as detonating cord or blasting caps. In primers made of solid materials, the passage is typically directly through the solid cast explosive. Thus, direct contact between this primer and the detonator within the passage is made possible. A problem is experienced in this regard when using aqueous explosives as primers, since a conventional primer cup cannot provide for the retention of aqueous explosives and yet have a through-passage permitting insertion and direct contact of the detonator to the aqueous explosive. Further, when moisture contacts certain primers used as nitromethane formulations or dynamite formulations, it renders them ineffective as blasting agents. Although primer cups are often lowered into water environments, conventional primer cups have been unable to provide for the exclusion of moisture from outside sources and yet have a through-passage permitting insertion and direct contact of the detonator to the moisture sensitive explosive.
In the past, some primer cups have permitted the detonator to be affixed external to the cup. These primer cups are particularly useful for containing such primers as granular TNT. However, the wall separating the detonator from the primer cup contents constitutes an additional barrier which the detonator explosion must penetrate before the primer material can be exploded. In order to overcome the detonation barrier, a high energy detonating cord must be used and/or the primer material must be made dangerously sensitive. This problem becomes even more acute when aqueous primers are utilized.
Still another problem among prior art primer cups is experienced while lowering them into the borehole. In practice, a borehole is often provided with several separate levels of explosive material. In these instances, it is desirable to place a primer for each of the separate levels of explosive material. To accomplish this, a primer is situated on a length of detonating cord and placed near the bottom of a borehole. A quantity of blasting agent is then placed into the borehole around the primer. Thereafter, another primer is dropped down along the same detonating cord. More specifically, the detonating cord is extended through the passage in the primer cup, and the cup is then permitted to descend the borehole along the cord until it rests upon the blasting agent previously placed therein. After several charges of blasting agent are in place and primed, the same detonating cord can be used to explode all the primers essentially simultaneously.
Some of the primer cups utilized in the past have been designed such that the passage through which the detonating cord passes is tapered from a wider opening at its lower end to a narrower opening at its top end. This tapering avoids problems in the molding process, but has resulted in difficulty in lowering the primer cups down the detonating cord. These primer cups often become lodged on the detonating cord or against the side of the borehole as a result of the draft of the passage in the cup. Further, debris upon the detonating cord may collect within the tapered passage causing the cup to hang up along the cord.
An additional problem is experienced if the passage through which the detonating cord passes does not extend through the center of the primer cup. In that case, the first primer cups dropped along the detonating cord may cause twisting and kinking in the detonating cord so that subsequent cups dropped along the detonating cord become entangled or stopped at the kinked portion.
It would, therefore, be a significant contribution to the prior art to provide a primer cup which may be sealed to provide easier handling of aqueous explosives, and to prevent moisture contamination of moisture sensitive explosives. It would also be an important contribution for such a cup to be structurally sound while providing a means by which relatively lower strength detonating cords or blasting caps could be used to produce explosions effectively penetrating the primer cup walls to initiate the explosion of the contained aqueous primer material. It would additionally be desirable to provide a primer cup which could be reliably lowered down a detonating cord with the substantial elimination of jamming due to the draft of the cup passage or debris on the cord.